This invention relates to the preparation of rigid closed-cell polyisocyanate-based foams, particularly polyurethane or polyurethane-modified polyiscyanurate foams where the use of hard CFC blowing agents is minimized or eliminated.
It is well-known to prepare foamed, closed-cell materials by the reaction of polyisocyanates with active hydrogen-containing materials in the presence of blowing agents. Typically these blowing agents are organic compounds which boil or decompose to give gaseous products at temperatures convenient for foam preparation. Especially useful in the preparation of rigid polyisocyanate-based foams are the halocarbon blowing agents, which, in addition to blowing, provide other physical properties, e.g., thermal insulation. The most commonly employed halocarbons are the Refrigerants R-11, R-12 and R-113, sometimes referred to as "hard" halocarbons.
The "hard" halocarbons are those in which all hydrogens of the alkane have been replaced by a halogen, normally fluorine or chlorine. These halocarbons, also called chlorofluorocarbons (CFC) are stable in the environment and are not degraded before reaching the upper strata of the atmosphere. At the upper stratum of the atmosphere, theoretically, they are thought to participate in, and change the free radical reactions and chemical equilibrium associated with the balance of the composition of the atmosphere, see "Ozone Studies in Antarctica" by J. Farman, New Scientist, p. 49 (Nov. 12, 1987). Accordingly, it is highly desirable to replace all hard blowing agents with alternative products which are not considered to change the composition and equilibriums of the upper atmosphere.
One possible means is to replace hard halocarbons with "soft" halocarbon blowing agents in the preparation of foams. Soft halocarbons are those which contain at least one hydrogen atom from the corresponding alkane in addition to the halogens, normally chlorine and fluorine. Such compounds are more readily degraded in the lower strata of the atmosphere and therefore significantly less reaches the upper strata of the atmosphere. Several "soft" CFCs exist which have boiling points and physical properties similar to the commonly used hard CFCs. The use of such blowing agents in the preparation of polyurethane foams has been discussed, see, for example, U.S. Pat. No. 4,636,529 and 4,076,644.
However, the use of soft CFCs in the preparation of polyurethane foam has been limited due to their restricted commercial availability and high pricing in comparison to the most commonly used hard CFCs.
An alternative means of replacing hard CFCs in the preparation of polyurethane foam is to use water. Water reacts with the isocyanate, generating carbon dioxide which then causes the reaction mixture to expand and attain a cellular structure. The use of water in the preparation of flexible polyurethane foams is well-established, where it comprises the major portion or all of the total blowing agent requirement. However, water has not successfully been used as the major or total portion of blowing agent in the preparation of rigid polyurethane foams. Frequently, when water is used at such levels, poor quality foams are obtained displaying a high degree of friability, low compressive strengths and high open-cell contents. In addition, the use of water has not until now been economically interesting due to the greatly increased consumption of costly isocyanate.
U.S. Pat. No. 4,273,882 teaches the preparation of polyurethane foams using water as the major portion of the blowing agent. The foams are prepared in the absence of catalyst and display a high open-cell content. U.S. Pat. No. 3,965,052 teaches a process for the preparation of low smoke-generating polyisocyanate foams. Polyols are reacted with at least three chemical equivalents of polyisocyanate in the presence of hard CFC and from two to six parts by weight water. Foams prepared with different quantities of water and less polyisocyanate are said to display poor rigidity, compressive strengths and shrink.
It is therefore desirable to provide a process for the preparation of rigid, closed-cell polyurethane foam or polyurea-modified polyurethane foam where water is used as the major portion of the blowing agent, thereby reducing or eliminating the need for hard chlorofluorocarbon blowing agents.